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A novel CNT encapsulated phase change material with enhanced thermal conductivity and photo-thermal conversion performance
Solar Energy Materials and Solar Cells 184 (2018) 82–90
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
A novel CNT encapsulated phase change material with enhanced thermal conductivity and photo-thermal conversion performance ⁎
Yanfeng Chena,b,1, Qi Zhangb,c,1, Xiaoyan Wend, Huibin Yina, , Jian Liua,
T
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a
Key Laboratory of Distributed Energy Systems of Guangdong Province, Department of Energy and Chemical Engineering, Dongguan University of Technology, Dongguan 523808, China b School of Chemistry and Chemical Engineering, South China University of Technology, China c Department of Biomedical Engineering, University of Minnesota Twin Cities, USA d Automotive Engineering Institute of Guangzhou Automobile Group Co., Ltd., China
A R T I C LE I N FO
A B S T R A C T
Keywords: Phase change material CNT Thermal conductivity Photo-thermal performance
Phase change material is one of the most promising materials in thermal energy storage systems. In this work, a new phase change composite of Stearic Acid@Multi-walled carbon nanotubes (SA@MWCNTs) is prepared by a simple vacuum absorption method. The as-prepared composite is determined by scanning electronic microscope (SEM), Fourier transformation infrared spectroscope (FT-IR) and X-ray diffractometer (XRD). The results show that outside diameters of the PCM composite are about 50–60 nm and Stearic Acid is encapsulated in the MWCNTs without chemical reaction. The DSC results indicate that the melting temperature and latent heat of the composite are 59.28 °C and 91.94 J g−1, respectively. The encapsulation ratio of Stearic Acid is calculated to be 47% from the results of the DSC measurements. The as-prepared SA@MWCNTs composite was dispersed into the water to form stable suspension, and the suspension shows remarkable photo-thermal conversion performance with temperature increases from 30 to 80 °C. The receiver efficiency of this new kind of heat transfer fluid maintain 85% in wide temperature range. Furthermore, the PCM composite could maintain its phase transition perfectly after 50 melting–freezing cycles, and no leakage of paraffin was observed by SEM. The high heat storage capability and excellent photo-thermal conversion performance of the composite enable it to be a potential material to store solar energy in practical applications.
1. Introduction Sustainable energy generation is one of the most important challenges facing society today [1]. The perceived shortage of fossil fuels as well as environmental considerations will constrain the use of fossil fuels in the future. Therefore, researchers are motivated to find alternative sources of energy. In recent years, the use of solar energy has had a remarkable edge [2]. Solar energy has been explored through solar thermal utilization, photovoltaic power generation, and so on [3,4]. Solar thermal utilization is the most popular application among them. In conventional solar thermal collectors, plates or tubes coated with a layer of selectively absorbing material are used to absorb solar energy, and then energy is carried away by working fluids in the form of heat [5,6]. In this case the efficiency is limited by not only how effective the absorber captures solar energy but also how effectively the heat is transferred to the working fluid. An approach that has been proposed to enhance the efficiency of collectors while simplifying the system is to
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1
Corresponding authors. E-mail addresses: [email protected] (H. Yin), [email protected] (J. Liu). The authors contributed equally to this work.
https://doi.org/10.1016/j.solmat.2018.04.034 Received 21 October 2017; Received in revised form 19 April 2018; Accepted 26 April 2018 Available online 10 May 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.
directly absorb the solar energy within the fluid volume, the so-called direct absorption solar collector. In the last century, black liquids containing millimeter to micrometer-sized particle were used as heat transfer fluid in solar collectors because of their excellent photothermal properties. However, the applications of these suspensions are limited because of severe abrasion, sedimentation, and plug problems of coarse particles. Recently, nanofluids have been applied as working fluids in direct solar collectors [1,7,8]. Nanofluid is a new class of heat transfer fluids containing stably suspended nano-sized particles, fibers, or tubes in the conventional heat transfer fluids such as water, engine oil, ethylene glycol, etc. [9–13]. Nanoparticles offer the potential of improving the radiative properties of liquids leading to an increase in the efficiency of direct absorption solar collectors. Several researchers have reported that nanofluids could effectively improve the solar energy utilization, especially carbon-based nanofluids for their excellent photothermal property. Zhu et al. reported that carbon black nanofluids of high-volume fraction had better photothermal properties and higher
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thermal conductivity and also exhibited a shear thinning behavior [14]. Sani et al. found that thermal properties and sunlight absorption of nanofluids consisting in aqueous suspensions of single wall carbon nanohorns were higher than pure water, make this new kind of nanofluids very interesting for increasing the overall efficiency of the sunlight exploiting device [15–17]. Poinern et al. showed that nanoparticles of functionalized carbon nanospheres had the potential to improve the photothermal properties of the working fluid [18]. To increase the utilization efficiency of solar energy, thermal energy storage is an important technology [19]. A lot of heat transfer fluids were developed to be applied in the direct solar thermal collectors, especially the low temperature heat transfer fluids such as water, ethylene glycol, conducting thermal energy storage with sensible heat storage technique. Compared with sensible heat storage techniques, latent heat storage technology using various phase change materials (PCMs) as the working media is particularly attractive due to its advantages of high energy storage density and isothermal nature during the phase change process. PCMs generally refers to materials having large latent heats of fusion with regards to melting and solidifying at a nearly constant temperature [20]. Most recently, PCMs have been recognized as being applicable to the sector of “high technology”, such as smart drug delivery [21], information storage [22], barcoding [23], and detection [24]. With PCMs dispersed in the heat transfer fluids, the heat can be quickly transferred to the PCMs, leading to an increase of the energy storage capacity of the heat transfer fluid. However, the direct utilization of the PCMs for heat storage is subject to some restrictions because of their leakage during the solid–liquid phase transitions and low thermal conductivity. To prevent the PCMs from interacting adversely with the heat transfer fluid, the PCMs are usually encapsulated by other robust materials [25]. There have been intensive efforts in exploring potential encapsulated methods for PCMs. For instance, the impregnation of paraffin in exfoliated graphite [26] or aerogel [27], the microencapsulation of PCMs [28]. Among these, microencapsulation is one of the most straightforward technology used in the heat transfer fluid. However, the preparation technology of the nanocapsule which is better for the heat transfer fluid preventing sedimentation is relatively difficult. Moreover, the most used organic polymers shell of the nanocapsule such as urea–formaldehyde resin [29,30] and polyurethane [31] have relatively lower thermal conductivity, decreasing the heat transfer performance of microencapsulated PCMs based slurry. Herein, a novel Stearic Acid@Multi-walled carbon nanotubes PCM composite was prepared by a simple vacuum absorption method. The as-prepared composite has an advisable encapsulated capacity of Stearic Acid, high thermal storage capability, and good thermal stability, and these new materials based heat transfer fluids are able to harvest visible light and convert it to thermal energy more effectively compared with traditional PCMs for latent heat thermal energy storage, indicating that the composite based novel kind of phase change slurry has great potential to store solar thermal energy in direct solar thermal collectors.
Fig. 1. Solar-driven transition diagram of the Stearic Acid@MWCNTs composite.
mixture of concentrated sulfuric acid and nitric acid (mass ratio = 3:1, 200 mL) and resuspending the nanotubes (30 g) in water followed by filtration with a cross-flow filtration system [32]. Finally, passing the resultant purified MWCNTs suspension though a polytetrafluoroethylene filter produced the purified MWCNTs. The Stearic Acid@MWCNTs composite PCM was prepared by using vacuum impregnation system, as shown in Fig. 1. MWCNTs was put in a conical flask heated in 90 °C, then the melted Stearic Acid was poured into it under vacuum. After 5 min, the untreated Stearic Acid@ MWCNTs was obtained. Then petroleum ether was used to wash the Stearic Acid stuck in the surface of the MWCNTs. Ultimately, the final Stearic Acid@MWCNTs composite PCM was received after filtration and desiccation. Then the prepared Stearic Acid@MWCNTs composites were formed into several round blocks by dry pressing with a homemade cylindrical mold (3 cm inside diameter and 1 cm height) under the pressure of 100 kg cm−2. Based on the volume of the mold we used, we calculated the masses of the composites needed for fabricating the blocks with the selected densities according to the formula m = ρ · V. Then, we added the calculated amounts of the composite PCM powders into the mold to fabricate the blocks. Finally, we measured the actual volumes and masses of the formed blocks to calculate their actual packing densities, which were 900.0, 1000.0, 1100.0, 1200.0, and 1300.0 kg m−3. The stable nanofluids was obtained by high-speed stirring the Stearic Acid@MWCNTs composite aqueous solution added a little surfactant. The surfactant were composed of OP80 and SPAN80 with the mass ratio of 1:1, and the mass fraction of surfactant to Stearic Acid@ MWCNTs composite was 1%. Three different nanofluids were prepared with different mass fraction of Stearic Acid@MWCNTs composite, which were 5, 10 and 15 wt%. 2.3. Characterizations of Stearic Acid@MWCNTs composite The morphology and microstructure of MWCNTs and Stearic Acid@ MWCNTs composite was observed by a transmission electron microscopy (FEI Tecnai G20). The structure of the composite was characterized by FT-IR spectra and X-ray diffraction. The FT-IR spectra were recorded on a Bruker 550 from 400 to 4000 cm−1 using KBr pellets, the X-ray diffraction (XRD) patterns of the samples were carried out on Xray diffractometer (D8 ADVANCE). The phase change temperature and latent heat of the samples were measured using a differential scanning calorimeter (Q20, TA). For DSC measurements, 5–8 mg for every sample was sealed in an aluminum pan for characterization at a heating rate of 10 °C min−1 under a constant stream of nitrogen at flow rate of 50 mL min−1. The thermal stability of MWCNTs and the Stearic Acid@ MWCNTs composite PCM was investigated by the thermogravimetric analysis (TGA) using a thermal analyzer (Q600 SDT, TA, URT100). The measurements were conducted by heating the samples from room temperature to 600 °C at a heating rate of 10 °C min−1 under nitrogen atmosphere with a flow rate of 100 mL min−1. The thermal conductivities of the obtained round blocks with
2. Experiment 2.1. Materials Stearic Acid (AR) was purchased from Shanghai RichJoint Chemical Reagents Co., Ltd. (CAS number: 57-11-4). MWCNTs (diameter inside: 30–50 nm, diameter outside: 50–60 nm, length: 5–30 µm, specific surface area: 120 m2 g−1) was purchased from Chengdu Organic Chemicals Co. Ltd. The surfactant OP80 and SPAN80 were purchased from Aladdin reagent (Shanghai) Co. LTD. 2.2. Preparation of Stearic Acid@MWCNTs composite and the composite based nanofluids We adopted a purification method that consists of refluxing in a 83
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to 90 °C for 5 min and then cooled to 30 °C for 5 min by a temperaturecontrolled instrument (C50p, HAAKE). This process was repeated for 50 heating-cooling cycles and the material was then evaluated for any changes in the morphology by TEM, heat storage capacity by DSC.
different packing densities were measured using a thermal constants analyzer (Hot Disk TPS 2500S, Hot Disk AB, Sweden). For comparison purpose, Stearic Acid was melted and poured into the cylindrical mold to fabricate two pieces of casting samples so as to measure the thermal conductivity of Stearic Acid. The photo-thermal conversion performance of Stearic Acid and compressed Stearic Acid@MWCNTs composite PCM with density of 900.0 kg m−3 were conducted under simulated solar irradiation. As shown in Fig. 1, the experimental apparatus consist of photo-thermal conversion system and data collection system. In this experiment, the samples were loaded in a quartzose beaker under the solar simulator (SOLAREDGE 700, Perfectlight), the heat storage was carried out when turn the light on. The power of the solar simulator was measured to be 1000 ± 20 W m−2. After the temperatures of the samples achieved ~ 75 °C, the light was turn off and the samples were immediately cooled at room temperature. The samples performed the heat release process at this time. The temperature of the samples during these periods were recorded by thermocouples and thermal camera. An irradiatometer (ST80C, Photoelectric Instrument Factory of Beijing Normal University) was used to measure and verify the power of the light irradiation from simulated light source.
3. Results and discussion 3.1. Characterization of Stearic Acid@MWCNTs composite The morphology of the pure MWCNTs and Stearic Acid@MWCNTs composite were characterized by transmission electron microscopy (TEM) as show in Fig. 3. The graphite layer and the empty cavities are clearly seen in the unfilled MWCNTs and the outside diameters of the pure MWCNTs are about 50–60 nm; particularly, the orifice of the MWCNTs were opened after purification (Fig. 3a, b). In Fig. 3c, d, continuously impregnated tubes are observed in MWCNTs; moreover, the surface of the tubes were smooth, indicating the encapsulation of Stearic Acid in the hollow interior of MWCNTs. To further confirm the tube structure of Stearic Acid@MWCNTs, the FT-IR and XRD spectrums of the phase change composite are conducted. The FT-IR spectra of Stearic Acid, MWCNTs and the composite PCM are displayed in Fig. 4a. In FT-IR spectra of SA, the peaks at 2919 and 2850 cm−1 cm are caused by stretching vibration of C–H and the peaks at 1705 and 1469 cm−1 are attributed to carboxylic acid (C==O) stretching vibration and symmetric carboxylate (COO–) stretching vibration, respectively [33,34]. For MWCNTs, the strong bands at ca. 1720 and 3440 cm−1 are due to the carboxyl groups. The symmetrical and asymmetrical bending vibrations of CH3 are represented at 1379 cm−1 [35]. In the spectrum of Stearic Acid@ MWCNTs composite, the peaks assigned to Stearic Acid at 2919, 1705 and 1469 cm−1, and the peaks assigned to MWCNTs at 1720 and 3440 cm−1 still existed, and no significant new peak is observed, which confirms that the tube structure of the Stearic Acid@MWCNTs composite, also reveals the composite material are physical interaction. The XRD patterns of the Stearic Acid, MWCNTs and Stearic Acid@ MWCNTs composite are displayed in Fig. 4b. In the pattern of Stearic Acid, The broad diffraction peak around 21.5° and 23.9° is ascribed to (110) peak of Stearic Acid [36]. In the pattern of MWCNTs, the strong diffraction peaks at 2θ = 26.4°, 44.5° and 55.2° were caused by regular crystallization of the MWCNTs, which are attributed to the diffractions of (002), (100) and (211) crystal planes [37]. The XRD pattern of the Stearic Acid@MWCNTs composite contains all the peaks of Stearic Acid and MWCNTs and no new peaks are produced, indicating that the composite is just the physical combination, no new substance has been produced.
2.4. Characterizations of Stearic Acid@MWCNTs composite based nanofluids The thermal conductivities of the obtained nanofluids was measured using a thermal constants analyzer (Hot Disk TPS 2500S, Hot Disk AB, Sweden). For comparison purpose, the thermal conductivity of the nanofluids at varying temperature was conducted with a thermostatic oil bath to control the temperature. In addition, the specific heat capacity was measured using a differential scanning calorimeter. A simulated solar irradiation was used to test the photo-thermal conversion performance of the Stearic Acid@MWCNTs Composite based nanofluids (Fig. 1b), which is used to test the photo-thermal conversion performance of the Stearic Acid@MWCNTs Composite. The same mass of the nanofluids with different mass fraction of Stearic Acid@MWCNTs composite were put into the quartzose beaker, the heat storage were carried out when turn the light on. The temperature of the samples during these periods were recorded by thermocouples. The effect of the mass fraction of the composite on the thermal conductivity, specific heat and the photo-thermal performance of the suspensions was investigated. The reciever efficiency was calculated by the formula (1) below.
η=
m ∫ CP (T ) dT Gs At
× 100%
(1)
Where η is the receiver efficiency, m represent the mass of the heat transfer fluid, Cp(T) is the specific heat of the heat transfer fluid, T is the temperature of the heat transfer fluid, Gs is the irradiance of the solar simulator, A is the surface area of the receiver, t is the time [33] (Fig. 2). To test the reversible stability of samples, the nanofluids was heated
3.2. Melting–freezing behavior of Stearic Acid@MWCNTs composite The phase change temperatures and latent heats of pure Stearic Acid and the encapsulated Stearic Acid@MWCNTs composite were measured using differential scanning calorimeter (DSC). Fig. 5a shows the melting–freezing DSC curves of the Stearic Acid and the Stearic Acid@ MWCNTs composite in the first phase change cycle. There are one endothermic peak in the melting DSC curve and one exothermic peak in the solidifying DSC curve, which were used to calculate the melting and freezing latent heat values, respectively. The melting and freezing temperatures are measured to be 61.22 and 71.08 °C for Stearic Acid and 59.28 and 70.65 °C for Stearic Acid@MWCNTs composite and the melting and freezing latent heats are measured to be 196.7 and 195.6 J g−1 for Stearic Acid and 91.94 and 92.55 J g−1 for the composite, respectively. The phase change characteristics of the Stearic Acid@MWCNTs composite are quite similar to those of Stearic Acid, because there is no chemical reaction between Stearic Acid and MWCNTs in preparation process. As one of the most important phase change performances affecting the working effect of the composite PCMs, the phase change enthalpy
Fig. 2. Solar-driven transition diagram of the Stearic Acid@MWCNTs composite based nanofluids for the light-to-heat device. 84
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Fig. 3. SEM images of MWCNTs (a, b) and Stearic Acid@MWCNTs composite (c, d).
the result of DSC measurement.
strongly depends on the encapsulation ratio of Stearic Acid absorbed in MWCNTs. The encapsulation ratio (R) can be calculated by the results of the DSC measurements and according to Eq. (2).
R=
ΔHm, Composite ΔHm, SA
× 100%
3.4. Reversible Stability of the Stearic Acid@MWCNTs composite To prove the phase change reversibility of the composites, heat/cool cycling tests were performed, and the results of a 200 cycle test are shown in Fig. 6. The tests were conducted in a temperature controlled oil bath (20–110–20 °C) to ensure the solid-liquid-solid cycles. After the heat/cool cycling tests, the composites were then checked with DSC for their latent heat storage capacity (Fig. 6a). Compared to 91.94 and 92.55 J g−1 before the cycles, the latent heats of melting and freezing were 91.47 and 90.75 J g−1 after 200 cycles, respectively. In addition, the excellent reversibility of the Stearic Acid@MWCNTs composites were observed, even after operating for 200 cycles. To further evaluate the reversible stability of the composites, we conducted the SEM observation on the Stearic Acid@MWCNTs composite after 200 heat/cool cycles. As shown in Fig. 6b, the morphology of the composites is similar to that before heat/cool cycles, implying that the composites did not leak in the heat/cool process. The stability of the nanofliuds (including 5%, 10% and 15% Stearic Acid@MWCNTs composites) were also investigated after 200 heat/cool cycles, and the result can be seen in Fig. 6c. It shows that there were no obvious sedimentation happened in the nanofliuds, which proved the good stability of the nanofluids.
(2)
Where ΔHm,Composite and ΔHm,SA represent the melting latent heat of the composite PCMs and Stearic Acid, respectively. The encapsulation ratio (R) of Stearic Acid in the composite PCMs is calculated to be 47.0% according to Table 1. 3.3. Thermal stability of Stearic Acid@MWCNTs composite The thermal stability of Stearic Acid, MWCNTs and the Stearic Acid@MWCNTs composite are studied by thermogravimetric analysis (TGA) and the curves of weight loss percentage are shown in Fig. 5b. The pure Stearic Acid starts to be removed at about 170 °C, and the final weight loss percentage is nearly 100% at 300 °C. For MWCNTs, there is no obvious weight loss until 500 °C and the final weight loss percentage is nearly 100%. There are two steps in the TGA curve of the Stearic Acid@MWCNTs composite, the first step is attributed to the weight loss of Stearic Acid and the second step is due to the vanishment of MWCNTs. Particularly, the composite starts to loss weight at ~ 170 °C and the weight loss percentage is nearly 50%. The second step of weight loss begins at 500 °C and the final weight loss percentage is nearly 100%. The thermogravimetric measurement also reveals that the weight percentage of the Stearic Acid is ~ 50%, which is matching with
3.5. Thermal conductivity of SA@MWCNTs composite The cylindrical compressed Stearic Acid@MWCNTs composite 85
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Table 1 Melting–freezing properties for pure Stearic Acid and Stearic Acid@CNT composite.
Stearic Acid Stearic Acid@CNT Stearic Acid@CNT (after 200 cycles)
Tm/°C
ΔHm/ J g−1
Tf/°C
ΔHf/J g−1
61.22 59.28 59.22
196.7 91.94 91.47
71.08 70.65 70.57
195.6 92.55 90.75
η/%
– 47.0 46.5
Fig. 4. FT-IR patterns (a) and XRD patterns (b) of the Stearic Acid, MWCNTs and Stearic Acid@MWCNTs composite.
Fig. 6. DSC curves for 200 phase change cycles of the Stearic Acid@MWCNTs composite (a), SEM image of Stearic Acid@MWCNTs composite after 200 phase change cycles (b)，sedimentation pictures of Stearic Acid@MWCNTs nanofluids after 200 phase change cycles (c).
PCMs were formed by dry pressing of PCM powders containing 47 wt% Stearic Acid in the homemade mold, whose packing densities are 900.0, 1000.0, 1100.0, 1200.0 and 1300.0 kg m−3, respectively. All the round blocks have smooth surface without any cracks, suggesting that the Stearic Acid@MWCNTs composite PCM has good formability, as shown in Fig. 7b (embedded in the bottom right). The thermal conductivities of the blocks have been measured by the
Fig. 5. Melting–freezing DSC curves of Stearic Acid and Stearic Acid@MWCNTs composite (a), TGA curves of Stearic Acid, MWCNTs, and Stearic Acid@ MWCNTs composite (b).
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thermal camera. As shown in Fig. 8 (left side), the red images represent the Stearic Acid@MWCNTs cylinder and the blue portion represent the external environment. It is clearly seen that upon solar irradiation the temperature of the composite PCM rapidly increased from 38.91 to 49.44 °C in 1.2 min, and then it maintained a temperature of less than 74 °C for 10.2 min, which is slower than before process, implying that the phase change composite storage large amount of thermal energy in the melting process. Similarly, when the Stearic Acid@MWCNTs composite at 74 °C was placed at 28 °C, the Stearic Acid@MWCNTs composite conducts the thermal release performance. The Stearic Acid@ MWCNTs cylinder could maintain a temperature of over 37 °C for more than 25 min. Above results indicate that the Stearic Acid@MWCNTs composite PCMs with excellent photo-thermal performance show great potential as direct solar light absorber for industrial thermal utilization, such as hot water heater, preheat burner. 3.7. Thermal property and photo-thermal performance of SA@MWCNTs composite based slurry The Stearic Acid@MWCNTs composite based phase change nanofluids is prepared by dispersing the composite into the water with tiny surfactant. The thermal conductivity of the H2O, 5 wt%, 10 wt% and 15 wt% PCMs suspension is shown in Fig. 9a. The thermal conductivity of the water increases from 0.590 to 0.668 W m−1 K−1 as the temperature range from 25 to 85 °C which is attributed to the enhancement of natural convection. When dispersing 5 wt% of the composite PCMs into the water, the thermal conductivity increases from 0.752 to 1.015 W m−1 K−1 in the same temperature range, which is 54.1% enhancement to the basefluid. The rise of the thermal conductivity as temperature increases is due to the intense natural convection of the nanofluids at high temperature. Particularly, the sharp increase between 55 °C and 75 °C is attributed to the melting of the composite PCMs. After the phase change process, the thermal conductivity at 85 °C is slightly lower than it at 75 °C but still higher than the nanofluids with equivalent mass ratio of CNTs, which is according to the Supplementary materials (S1). Similarly, the same variation trend are obtained when the composite PCMs increase to 10 wt% or 15 wt%, with the thermal conductivity enhancement of 62.0% or 72.2%. The thermal conductivity enhancement increases with the mass fraction of the composite PCMs which is attributed to the higher thermal conductivities of composite PCMs. The specific heat of the H2O, 5 wt%, 10 wt% and 15 wt% PCMs suspension is shown in Fig. 9b. The specific heat of water slightly increases from 4.205 to 4.305 J g−1 K−1 as the temperature range from 40 to 85 °C. When dispersing composite PCMs into the water, the specific heat was largely enhanced in the melting process. For instance, the 5 wt% composite PCMs based nanofluids range from 4.200 to 4.895 J g−1 K−1 in the melting process with an enhancement of 15.2%. Similarly, with the increasement of the mass fraction of the composite PCMs, the specific heat enhancement reaches up to 47.1% when the mass fraction of the composite PCMs is 15%. By contrast, the specific heat of the nanofluids with equivalent mass ratio of CNTs can be seen in the Supplementary materials (S2). The specific heat of the nanofluids without PCMs wasn't largely enhanced in the melting process. It is indicated that the heat storage capacity of the heat transfer fluid could be enhanced by adding the composite PCMs, because the composite PCMs have high energy capacity in the melting and solidifying process, as shown in Fig. 5a. Fig. 9c shows the incident photon-to-thermal conversion spectra of the composites/water based direct solar thermal collectors. During the heating process, it took 7000 s for the 5 wt% PCMs nanofluids to raise the temperature from room temperature (about 30 °C) to 75 °C. While under the same exposure time, the temperature of the 10 wt% PCMs nanofluids raise to 80 °C. That is attributed to the high mass ratio of the CNTs. Notably, the temperature of the 15 wt% PCMs nanofluids just goes up to 65 °C, which is due to reflection of the higher mass ratio of
Fig. 7. Thermal conductivity of the Stearic Acid and Stearic Acid@MWCNTs composite (a) thermal conductivity of the Stearic Acid@MWCNTs composite vis density (b).
transient plane source method, an advanced technique evolving from the hot wire method. The thermal conductivity of the Stearic Acid casting sample with the density of 847.0 kg m−3 is measured to be 0.256 W m−1 K−1 (as shown in Fig. 7a), which is very lower than the Stearic Acid@MWCNTs round blocks with the density of 900.0 kg m−3. Fig. 7b illustrates the thermal conductivity of the Stearic Acid@ MWCNTs round blocks with different packing densities. As the packing density of the blocks is increased from 900.0, 1000.0, 1100.0 and 1200.0 to 1300.0 kg m−3, their thermal conductivity accordingly increases from 7.159, 8.215, 9.134, and 10.058 to 11.154 W m−1 K−1. The relationship between the packing density (x) and the thermal conductivity (y) of the Stearic Acid@MWCNTs round blocks can be fitted into a linear equation, indicating an obvious increase in the thermal conductivity with the packing density. Note that all the Stearic Acid@MWCNTs composite PCM round blocks have much higher thermal conductivity than the Stearic Acid casting sample, owing to the integration of Stearic Acid with MWCNTs that has superior thermal conductivity. 3.6. Photo-thermal performance of SA@MWCNTs composite The photo-thermal performance of the formable Stearic Acid@ MWCNTs composite PCM containing 47 wt% of Stearic Acid with the density of 900.0 kg m−3 was conducted in the homemade equipment as shown in Fig. 1. Fig. 8 (bottom right corner) shows the photo-thermal conversion spectra of the Stearic Acid and Stearic Acid@MWCNTs composite PCM. Compared with the Stearic Acid, the temperature of the composite PCM rapidly increased upon solar irradiation. During the heating process, it took 1960 s for Stearic Acid to raise the temperature from room temperature to 75 °C but only 716 s for the Stearic Acid@ MWCNTs composite PCM. This behavior is ascribed to the function of MWCNTs as an effective photon captor and molecular heater. Simultaneously, the photo-thermal performance of the formable Stearic Acid@MWCNTs composite PCM was observed clearly through a 87
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Fig. 8. Representative thermal images about the photo-thermal performance of the Stearic Acid@MWCNTs composite.
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Fig. 9. Thermal conductivity of Stearic Acid@MWCNTs composite based nanofluids (a), Specific heat of Stearic Acid@MWCNTs composite based nanofluids (b), Photo-thermal performance of the Stearic Acid@MWCNTs composite based nanofluids (c), (d), receiver efficiency of the Stearic Acid@MWCNTs composite based nanofluids (f).
indicate that adding the encapsulated PCMs into the water can not only increase the thermal conductivity and heat capacity of the fluids, but also improve the photo-thermal performance as the heat transfer fluid in the direct solar thermal receiver. Moreover, above results show that the water based phase change slurry has the great potential to be used as low temperature heat transfer fluids in the direct absorption solar collector.
the PCMs nanofluids. Compared with the Stearic Acid@MWCNTs composite based nanofluids, without the light absorption material (MWCNTs), there is no obvious temperature rise for the basefluid. This result also proved the function of MWCNTs in this nanofluids. When dispersed the same mass fraction of CNTs into the water, the photo-thermal performance of the receiver could be almost alike (Figs. 9d, S3). This behavior is ascribed to the function of CNTs as an effective photon captor and molecular heater. To further investigate the photo-thermal performance, the receiver efficiency of above samples based direct solar thermal collectors was calculated by formula 1 according to the results of Fig. 9c. When dispersing PCMs into the fluids, their receiver efficiency could maintain in a wide temperature range, as shown in Fig. 9f. The 10 wt% PCMs based heat transfer fluid show higher receiver efficiency than the equivalent mass ratio of CNTs sample without Stearic Acid which is ascribed to the high specific heat of the PCMs based heat transfer fluid. Similarly, the receiver efficienc of 5 wt% and 15 wt% PCMs based heat transfer fluid could also maintain in a wide temperature range (Fig. 8e) and the heat transfer fluid with equivalent mass ratio of CNTs were also lower than them (Fig. S4). Particularly, the lower receiver efficienc of 15 wt% is also due to reflection of the higher mass ratio of the PCMs nanofluids. Above results
4. Conclusion A photo-driven encapsulated Stearic Acid@MWCNTs composites was successfully prepared by an simple vacuum impregnation process. In the composite, the Stearic Acid was used as the material for thermal energy storage, and MWCNTs acted as the Supporting material for improving the thermal conductivity and photo-thermal conversion performance of the composite. The melting temperature and latent heat of the composite were determined as 59.28 °C and 91.94 J g−1, respectively. The phase change characteristics of the composite are close to those of the pure Stearic Acid because there is no chemical reaction between Stearic Acid and MWCNTs in preparation of the composite. The composite with the Stearic Acid encapsulation ratio of 47% could 89
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maintain its phase transition perfectly after subjected to 200 melting–freezing cycles. Then the as-prepared Stearic Acid@MWCNTs composite was dispersed into water to form novel heat transfer fluid (with mass fraction of 5%, 10% and 15%) to be used in the direct solar thermal receiver. This new kind of fluid shows enhanced thermal conductivity (2 times), specific heat (1.5 times) and photo-thermal performance (there is no obvious temperature rise for the basefluid) as compared to the basefluids. Our work indicating that the water based phase change slurry has great potential to be used as low temperature heat transfer fluids in the solar thermal utilization systems.
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Acknowledgement This work was supported by the Guangdong Natural Science Foundation (2015A030313653) and the Guangdong Province Science and Technology Plan Project (2017A010103047). Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2018.04.034. References [1] T.P. Otanicar, P.E. Phelan, R.S. Prasher, G. Rosengarten, R.A. Taylor, Nanofluidbased direct absorption solar collector, J. Renew. Sustain. Energy 2 (2010) 033102. [2] O. Mahian, A. Kianifar, S.A. Kalogirou, I. Pop, S. Wongwises, A review of the applications of nanofluids in solar energy, Int. J. Heat Mass Transf. 57 (2013) 582–594. [3] Y. Tripanagnostopoulos, Aspects and improvements of hybrid photovoltaic/thermal solar energy systems, Sol. Energy 81 (2007) 1117–1131. [4] P. Charalambous, G. Maidment, S. Kalogirou, K. Yiakoumetti, Photovoltaic thermal (PV/T) collectors: a review, Appl. Therm. Eng. 27 (2007) 275–286. [5] T.P. Otanicar, J.S. Golden, Comparative environmental and economic analysis of conventional and nanofluid solar hot water technologies, Environ. Sci. Technol. 43 (2009) 6082–6087. [6] L. Mu, Q. Zhu, L. Si, Radiative properties of nanofluids and performance of a direct solar absorber using nanofluids, in: Proceedings of the ASME 2009s International Conference on Micro/Nanoscale Heat and Mass Transfer, American Society of Mechanical Engineers, 2009, pp. 549–553. [7] T.P. Otanicar, P.E. Phelan, J.S. Golden, Optical properties of liquids for direct absorption solar thermal energy systems, Sol. Energy 83 (2009) 969–977. [8] C. Shou, Z. Luo, T. Wang, J. Cai, J. Zhao, M. Ni, K. Cen, Research on the application of nano-fluids into the solar photoelectric utilization, Shanghai Electr. Power 16 (2009) 8–12. [9] H. Zhu, S. Liu, L. Xu, C. Zhang, Preparation, characterization and thermal properties of nanofluids, in: Donald M. Sabatini (Ed.), Leading Edge Nanotechnology Research Developments, 2007, pp. 5–38. [10] L. Wang, J. Fan, Nanofluids research: key issues, Nanoscale Res. Lett. 5 (2010) 1241–1252. [11] D. Wu, H. Zhu, L. Wang, L. Liu, Critical issues in nanofluids preparation, characterization and thermal conductivity, Curr. Nanosci. 5 (2009) 103–112. [12] S.K. Das, S.U. Choi, W. Yu, T. Pradeep, Nanofluids: Science and Technology, John Wiley & Sons, 2007. [13] S. Chol, Enhancing Thermal Conductivity of Fluids with Nanoparticles 231 ASMEPublications-Fed, 1995, pp. 99–106. [14] D. Han, Z. Meng, D. Wu, C. Zhang, H. Zhu, Thermal properties of carbon black aqueous nanofluids for solar absorption, Nanoscale Res. Lett. 6 (2011) 1–7.
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
A novel CNT encapsulated phase change material with enhanced thermal conductivity and photo-thermal conversion performance ⁎
Yanfeng Chena,b,1, Qi Zhangb,c,1, Xiaoyan Wend, Huibin Yina, , Jian Liua,
T
⁎
a
Key Laboratory of Distributed Energy Systems of Guangdong Province, Department of Energy and Chemical Engineering, Dongguan University of Technology, Dongguan 523808, China b School of Chemistry and Chemical Engineering, South China University of Technology, China c Department of Biomedical Engineering, University of Minnesota Twin Cities, USA d Automotive Engineering Institute of Guangzhou Automobile Group Co., Ltd., China
A R T I C LE I N FO
A B S T R A C T
Keywords: Phase change material CNT Thermal conductivity Photo-thermal performance
Phase change material is one of the most promising materials in thermal energy storage systems. In this work, a new phase change composite of Stearic Acid@Multi-walled carbon nanotubes (SA@MWCNTs) is prepared by a simple vacuum absorption method. The as-prepared composite is determined by scanning electronic microscope (SEM), Fourier transformation infrared spectroscope (FT-IR) and X-ray diffractometer (XRD). The results show that outside diameters of the PCM composite are about 50–60 nm and Stearic Acid is encapsulated in the MWCNTs without chemical reaction. The DSC results indicate that the melting temperature and latent heat of the composite are 59.28 °C and 91.94 J g−1, respectively. The encapsulation ratio of Stearic Acid is calculated to be 47% from the results of the DSC measurements. The as-prepared SA@MWCNTs composite was dispersed into the water to form stable suspension, and the suspension shows remarkable photo-thermal conversion performance with temperature increases from 30 to 80 °C. The receiver efficiency of this new kind of heat transfer fluid maintain 85% in wide temperature range. Furthermore, the PCM composite could maintain its phase transition perfectly after 50 melting–freezing cycles, and no leakage of paraffin was observed by SEM. The high heat storage capability and excellent photo-thermal conversion performance of the composite enable it to be a potential material to store solar energy in practical applications.
1. Introduction Sustainable energy generation is one of the most important challenges facing society today [1]. The perceived shortage of fossil fuels as well as environmental considerations will constrain the use of fossil fuels in the future. Therefore, researchers are motivated to find alternative sources of energy. In recent years, the use of solar energy has had a remarkable edge [2]. Solar energy has been explored through solar thermal utilization, photovoltaic power generation, and so on [3,4]. Solar thermal utilization is the most popular application among them. In conventional solar thermal collectors, plates or tubes coated with a layer of selectively absorbing material are used to absorb solar energy, and then energy is carried away by working fluids in the form of heat [5,6]. In this case the efficiency is limited by not only how effective the absorber captures solar energy but also how effectively the heat is transferred to the working fluid. An approach that has been proposed to enhance the efficiency of collectors while simplifying the system is to
⁎
1
Corresponding authors. E-mail addresses: [email protected] (H. Yin), [email protected] (J. Liu). The authors contributed equally to this work.
https://doi.org/10.1016/j.solmat.2018.04.034 Received 21 October 2017; Received in revised form 19 April 2018; Accepted 26 April 2018 Available online 10 May 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.
directly absorb the solar energy within the fluid volume, the so-called direct absorption solar collector. In the last century, black liquids containing millimeter to micrometer-sized particle were used as heat transfer fluid in solar collectors because of their excellent photothermal properties. However, the applications of these suspensions are limited because of severe abrasion, sedimentation, and plug problems of coarse particles. Recently, nanofluids have been applied as working fluids in direct solar collectors [1,7,8]. Nanofluid is a new class of heat transfer fluids containing stably suspended nano-sized particles, fibers, or tubes in the conventional heat transfer fluids such as water, engine oil, ethylene glycol, etc. [9–13]. Nanoparticles offer the potential of improving the radiative properties of liquids leading to an increase in the efficiency of direct absorption solar collectors. Several researchers have reported that nanofluids could effectively improve the solar energy utilization, especially carbon-based nanofluids for their excellent photothermal property. Zhu et al. reported that carbon black nanofluids of high-volume fraction had better photothermal properties and higher
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thermal conductivity and also exhibited a shear thinning behavior [14]. Sani et al. found that thermal properties and sunlight absorption of nanofluids consisting in aqueous suspensions of single wall carbon nanohorns were higher than pure water, make this new kind of nanofluids very interesting for increasing the overall efficiency of the sunlight exploiting device [15–17]. Poinern et al. showed that nanoparticles of functionalized carbon nanospheres had the potential to improve the photothermal properties of the working fluid [18]. To increase the utilization efficiency of solar energy, thermal energy storage is an important technology [19]. A lot of heat transfer fluids were developed to be applied in the direct solar thermal collectors, especially the low temperature heat transfer fluids such as water, ethylene glycol, conducting thermal energy storage with sensible heat storage technique. Compared with sensible heat storage techniques, latent heat storage technology using various phase change materials (PCMs) as the working media is particularly attractive due to its advantages of high energy storage density and isothermal nature during the phase change process. PCMs generally refers to materials having large latent heats of fusion with regards to melting and solidifying at a nearly constant temperature [20]. Most recently, PCMs have been recognized as being applicable to the sector of “high technology”, such as smart drug delivery [21], information storage [22], barcoding [23], and detection [24]. With PCMs dispersed in the heat transfer fluids, the heat can be quickly transferred to the PCMs, leading to an increase of the energy storage capacity of the heat transfer fluid. However, the direct utilization of the PCMs for heat storage is subject to some restrictions because of their leakage during the solid–liquid phase transitions and low thermal conductivity. To prevent the PCMs from interacting adversely with the heat transfer fluid, the PCMs are usually encapsulated by other robust materials [25]. There have been intensive efforts in exploring potential encapsulated methods for PCMs. For instance, the impregnation of paraffin in exfoliated graphite [26] or aerogel [27], the microencapsulation of PCMs [28]. Among these, microencapsulation is one of the most straightforward technology used in the heat transfer fluid. However, the preparation technology of the nanocapsule which is better for the heat transfer fluid preventing sedimentation is relatively difficult. Moreover, the most used organic polymers shell of the nanocapsule such as urea–formaldehyde resin [29,30] and polyurethane [31] have relatively lower thermal conductivity, decreasing the heat transfer performance of microencapsulated PCMs based slurry. Herein, a novel Stearic Acid@Multi-walled carbon nanotubes PCM composite was prepared by a simple vacuum absorption method. The as-prepared composite has an advisable encapsulated capacity of Stearic Acid, high thermal storage capability, and good thermal stability, and these new materials based heat transfer fluids are able to harvest visible light and convert it to thermal energy more effectively compared with traditional PCMs for latent heat thermal energy storage, indicating that the composite based novel kind of phase change slurry has great potential to store solar thermal energy in direct solar thermal collectors.
Fig. 1. Solar-driven transition diagram of the Stearic Acid@MWCNTs composite.
mixture of concentrated sulfuric acid and nitric acid (mass ratio = 3:1, 200 mL) and resuspending the nanotubes (30 g) in water followed by filtration with a cross-flow filtration system [32]. Finally, passing the resultant purified MWCNTs suspension though a polytetrafluoroethylene filter produced the purified MWCNTs. The Stearic Acid@MWCNTs composite PCM was prepared by using vacuum impregnation system, as shown in Fig. 1. MWCNTs was put in a conical flask heated in 90 °C, then the melted Stearic Acid was poured into it under vacuum. After 5 min, the untreated Stearic Acid@ MWCNTs was obtained. Then petroleum ether was used to wash the Stearic Acid stuck in the surface of the MWCNTs. Ultimately, the final Stearic Acid@MWCNTs composite PCM was received after filtration and desiccation. Then the prepared Stearic Acid@MWCNTs composites were formed into several round blocks by dry pressing with a homemade cylindrical mold (3 cm inside diameter and 1 cm height) under the pressure of 100 kg cm−2. Based on the volume of the mold we used, we calculated the masses of the composites needed for fabricating the blocks with the selected densities according to the formula m = ρ · V. Then, we added the calculated amounts of the composite PCM powders into the mold to fabricate the blocks. Finally, we measured the actual volumes and masses of the formed blocks to calculate their actual packing densities, which were 900.0, 1000.0, 1100.0, 1200.0, and 1300.0 kg m−3. The stable nanofluids was obtained by high-speed stirring the Stearic Acid@MWCNTs composite aqueous solution added a little surfactant. The surfactant were composed of OP80 and SPAN80 with the mass ratio of 1:1, and the mass fraction of surfactant to Stearic Acid@ MWCNTs composite was 1%. Three different nanofluids were prepared with different mass fraction of Stearic Acid@MWCNTs composite, which were 5, 10 and 15 wt%. 2.3. Characterizations of Stearic Acid@MWCNTs composite The morphology and microstructure of MWCNTs and Stearic Acid@ MWCNTs composite was observed by a transmission electron microscopy (FEI Tecnai G20). The structure of the composite was characterized by FT-IR spectra and X-ray diffraction. The FT-IR spectra were recorded on a Bruker 550 from 400 to 4000 cm−1 using KBr pellets, the X-ray diffraction (XRD) patterns of the samples were carried out on Xray diffractometer (D8 ADVANCE). The phase change temperature and latent heat of the samples were measured using a differential scanning calorimeter (Q20, TA). For DSC measurements, 5–8 mg for every sample was sealed in an aluminum pan for characterization at a heating rate of 10 °C min−1 under a constant stream of nitrogen at flow rate of 50 mL min−1. The thermal stability of MWCNTs and the Stearic Acid@ MWCNTs composite PCM was investigated by the thermogravimetric analysis (TGA) using a thermal analyzer (Q600 SDT, TA, URT100). The measurements were conducted by heating the samples from room temperature to 600 °C at a heating rate of 10 °C min−1 under nitrogen atmosphere with a flow rate of 100 mL min−1. The thermal conductivities of the obtained round blocks with
2. Experiment 2.1. Materials Stearic Acid (AR) was purchased from Shanghai RichJoint Chemical Reagents Co., Ltd. (CAS number: 57-11-4). MWCNTs (diameter inside: 30–50 nm, diameter outside: 50–60 nm, length: 5–30 µm, specific surface area: 120 m2 g−1) was purchased from Chengdu Organic Chemicals Co. Ltd. The surfactant OP80 and SPAN80 were purchased from Aladdin reagent (Shanghai) Co. LTD. 2.2. Preparation of Stearic Acid@MWCNTs composite and the composite based nanofluids We adopted a purification method that consists of refluxing in a 83
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to 90 °C for 5 min and then cooled to 30 °C for 5 min by a temperaturecontrolled instrument (C50p, HAAKE). This process was repeated for 50 heating-cooling cycles and the material was then evaluated for any changes in the morphology by TEM, heat storage capacity by DSC.
different packing densities were measured using a thermal constants analyzer (Hot Disk TPS 2500S, Hot Disk AB, Sweden). For comparison purpose, Stearic Acid was melted and poured into the cylindrical mold to fabricate two pieces of casting samples so as to measure the thermal conductivity of Stearic Acid. The photo-thermal conversion performance of Stearic Acid and compressed Stearic Acid@MWCNTs composite PCM with density of 900.0 kg m−3 were conducted under simulated solar irradiation. As shown in Fig. 1, the experimental apparatus consist of photo-thermal conversion system and data collection system. In this experiment, the samples were loaded in a quartzose beaker under the solar simulator (SOLAREDGE 700, Perfectlight), the heat storage was carried out when turn the light on. The power of the solar simulator was measured to be 1000 ± 20 W m−2. After the temperatures of the samples achieved ~ 75 °C, the light was turn off and the samples were immediately cooled at room temperature. The samples performed the heat release process at this time. The temperature of the samples during these periods were recorded by thermocouples and thermal camera. An irradiatometer (ST80C, Photoelectric Instrument Factory of Beijing Normal University) was used to measure and verify the power of the light irradiation from simulated light source.
3. Results and discussion 3.1. Characterization of Stearic Acid@MWCNTs composite The morphology of the pure MWCNTs and Stearic Acid@MWCNTs composite were characterized by transmission electron microscopy (TEM) as show in Fig. 3. The graphite layer and the empty cavities are clearly seen in the unfilled MWCNTs and the outside diameters of the pure MWCNTs are about 50–60 nm; particularly, the orifice of the MWCNTs were opened after purification (Fig. 3a, b). In Fig. 3c, d, continuously impregnated tubes are observed in MWCNTs; moreover, the surface of the tubes were smooth, indicating the encapsulation of Stearic Acid in the hollow interior of MWCNTs. To further confirm the tube structure of Stearic Acid@MWCNTs, the FT-IR and XRD spectrums of the phase change composite are conducted. The FT-IR spectra of Stearic Acid, MWCNTs and the composite PCM are displayed in Fig. 4a. In FT-IR spectra of SA, the peaks at 2919 and 2850 cm−1 cm are caused by stretching vibration of C–H and the peaks at 1705 and 1469 cm−1 are attributed to carboxylic acid (C==O) stretching vibration and symmetric carboxylate (COO–) stretching vibration, respectively [33,34]. For MWCNTs, the strong bands at ca. 1720 and 3440 cm−1 are due to the carboxyl groups. The symmetrical and asymmetrical bending vibrations of CH3 are represented at 1379 cm−1 [35]. In the spectrum of Stearic Acid@ MWCNTs composite, the peaks assigned to Stearic Acid at 2919, 1705 and 1469 cm−1, and the peaks assigned to MWCNTs at 1720 and 3440 cm−1 still existed, and no significant new peak is observed, which confirms that the tube structure of the Stearic Acid@MWCNTs composite, also reveals the composite material are physical interaction. The XRD patterns of the Stearic Acid, MWCNTs and Stearic Acid@ MWCNTs composite are displayed in Fig. 4b. In the pattern of Stearic Acid, The broad diffraction peak around 21.5° and 23.9° is ascribed to (110) peak of Stearic Acid [36]. In the pattern of MWCNTs, the strong diffraction peaks at 2θ = 26.4°, 44.5° and 55.2° were caused by regular crystallization of the MWCNTs, which are attributed to the diffractions of (002), (100) and (211) crystal planes [37]. The XRD pattern of the Stearic Acid@MWCNTs composite contains all the peaks of Stearic Acid and MWCNTs and no new peaks are produced, indicating that the composite is just the physical combination, no new substance has been produced.
2.4. Characterizations of Stearic Acid@MWCNTs composite based nanofluids The thermal conductivities of the obtained nanofluids was measured using a thermal constants analyzer (Hot Disk TPS 2500S, Hot Disk AB, Sweden). For comparison purpose, the thermal conductivity of the nanofluids at varying temperature was conducted with a thermostatic oil bath to control the temperature. In addition, the specific heat capacity was measured using a differential scanning calorimeter. A simulated solar irradiation was used to test the photo-thermal conversion performance of the Stearic Acid@MWCNTs Composite based nanofluids (Fig. 1b), which is used to test the photo-thermal conversion performance of the Stearic Acid@MWCNTs Composite. The same mass of the nanofluids with different mass fraction of Stearic Acid@MWCNTs composite were put into the quartzose beaker, the heat storage were carried out when turn the light on. The temperature of the samples during these periods were recorded by thermocouples. The effect of the mass fraction of the composite on the thermal conductivity, specific heat and the photo-thermal performance of the suspensions was investigated. The reciever efficiency was calculated by the formula (1) below.
η=
m ∫ CP (T ) dT Gs At
× 100%
(1)
Where η is the receiver efficiency, m represent the mass of the heat transfer fluid, Cp(T) is the specific heat of the heat transfer fluid, T is the temperature of the heat transfer fluid, Gs is the irradiance of the solar simulator, A is the surface area of the receiver, t is the time [33] (Fig. 2). To test the reversible stability of samples, the nanofluids was heated
3.2. Melting–freezing behavior of Stearic Acid@MWCNTs composite The phase change temperatures and latent heats of pure Stearic Acid and the encapsulated Stearic Acid@MWCNTs composite were measured using differential scanning calorimeter (DSC). Fig. 5a shows the melting–freezing DSC curves of the Stearic Acid and the Stearic Acid@ MWCNTs composite in the first phase change cycle. There are one endothermic peak in the melting DSC curve and one exothermic peak in the solidifying DSC curve, which were used to calculate the melting and freezing latent heat values, respectively. The melting and freezing temperatures are measured to be 61.22 and 71.08 °C for Stearic Acid and 59.28 and 70.65 °C for Stearic Acid@MWCNTs composite and the melting and freezing latent heats are measured to be 196.7 and 195.6 J g−1 for Stearic Acid and 91.94 and 92.55 J g−1 for the composite, respectively. The phase change characteristics of the Stearic Acid@MWCNTs composite are quite similar to those of Stearic Acid, because there is no chemical reaction between Stearic Acid and MWCNTs in preparation process. As one of the most important phase change performances affecting the working effect of the composite PCMs, the phase change enthalpy
Fig. 2. Solar-driven transition diagram of the Stearic Acid@MWCNTs composite based nanofluids for the light-to-heat device. 84
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Fig. 3. SEM images of MWCNTs (a, b) and Stearic Acid@MWCNTs composite (c, d).
the result of DSC measurement.
strongly depends on the encapsulation ratio of Stearic Acid absorbed in MWCNTs. The encapsulation ratio (R) can be calculated by the results of the DSC measurements and according to Eq. (2).
R=
ΔHm, Composite ΔHm, SA
× 100%
3.4. Reversible Stability of the Stearic Acid@MWCNTs composite To prove the phase change reversibility of the composites, heat/cool cycling tests were performed, and the results of a 200 cycle test are shown in Fig. 6. The tests were conducted in a temperature controlled oil bath (20–110–20 °C) to ensure the solid-liquid-solid cycles. After the heat/cool cycling tests, the composites were then checked with DSC for their latent heat storage capacity (Fig. 6a). Compared to 91.94 and 92.55 J g−1 before the cycles, the latent heats of melting and freezing were 91.47 and 90.75 J g−1 after 200 cycles, respectively. In addition, the excellent reversibility of the Stearic Acid@MWCNTs composites were observed, even after operating for 200 cycles. To further evaluate the reversible stability of the composites, we conducted the SEM observation on the Stearic Acid@MWCNTs composite after 200 heat/cool cycles. As shown in Fig. 6b, the morphology of the composites is similar to that before heat/cool cycles, implying that the composites did not leak in the heat/cool process. The stability of the nanofliuds (including 5%, 10% and 15% Stearic Acid@MWCNTs composites) were also investigated after 200 heat/cool cycles, and the result can be seen in Fig. 6c. It shows that there were no obvious sedimentation happened in the nanofliuds, which proved the good stability of the nanofluids.
(2)
Where ΔHm,Composite and ΔHm,SA represent the melting latent heat of the composite PCMs and Stearic Acid, respectively. The encapsulation ratio (R) of Stearic Acid in the composite PCMs is calculated to be 47.0% according to Table 1. 3.3. Thermal stability of Stearic Acid@MWCNTs composite The thermal stability of Stearic Acid, MWCNTs and the Stearic Acid@MWCNTs composite are studied by thermogravimetric analysis (TGA) and the curves of weight loss percentage are shown in Fig. 5b. The pure Stearic Acid starts to be removed at about 170 °C, and the final weight loss percentage is nearly 100% at 300 °C. For MWCNTs, there is no obvious weight loss until 500 °C and the final weight loss percentage is nearly 100%. There are two steps in the TGA curve of the Stearic Acid@MWCNTs composite, the first step is attributed to the weight loss of Stearic Acid and the second step is due to the vanishment of MWCNTs. Particularly, the composite starts to loss weight at ~ 170 °C and the weight loss percentage is nearly 50%. The second step of weight loss begins at 500 °C and the final weight loss percentage is nearly 100%. The thermogravimetric measurement also reveals that the weight percentage of the Stearic Acid is ~ 50%, which is matching with
3.5. Thermal conductivity of SA@MWCNTs composite The cylindrical compressed Stearic Acid@MWCNTs composite 85
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Table 1 Melting–freezing properties for pure Stearic Acid and Stearic Acid@CNT composite.
Stearic Acid Stearic Acid@CNT Stearic Acid@CNT (after 200 cycles)
Tm/°C
ΔHm/ J g−1
Tf/°C
ΔHf/J g−1
61.22 59.28 59.22
196.7 91.94 91.47
71.08 70.65 70.57
195.6 92.55 90.75
η/%
– 47.0 46.5
Fig. 4. FT-IR patterns (a) and XRD patterns (b) of the Stearic Acid, MWCNTs and Stearic Acid@MWCNTs composite.
Fig. 6. DSC curves for 200 phase change cycles of the Stearic Acid@MWCNTs composite (a), SEM image of Stearic Acid@MWCNTs composite after 200 phase change cycles (b)，sedimentation pictures of Stearic Acid@MWCNTs nanofluids after 200 phase change cycles (c).
PCMs were formed by dry pressing of PCM powders containing 47 wt% Stearic Acid in the homemade mold, whose packing densities are 900.0, 1000.0, 1100.0, 1200.0 and 1300.0 kg m−3, respectively. All the round blocks have smooth surface without any cracks, suggesting that the Stearic Acid@MWCNTs composite PCM has good formability, as shown in Fig. 7b (embedded in the bottom right). The thermal conductivities of the blocks have been measured by the
Fig. 5. Melting–freezing DSC curves of Stearic Acid and Stearic Acid@MWCNTs composite (a), TGA curves of Stearic Acid, MWCNTs, and Stearic Acid@ MWCNTs composite (b).
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thermal camera. As shown in Fig. 8 (left side), the red images represent the Stearic Acid@MWCNTs cylinder and the blue portion represent the external environment. It is clearly seen that upon solar irradiation the temperature of the composite PCM rapidly increased from 38.91 to 49.44 °C in 1.2 min, and then it maintained a temperature of less than 74 °C for 10.2 min, which is slower than before process, implying that the phase change composite storage large amount of thermal energy in the melting process. Similarly, when the Stearic Acid@MWCNTs composite at 74 °C was placed at 28 °C, the Stearic Acid@MWCNTs composite conducts the thermal release performance. The Stearic Acid@ MWCNTs cylinder could maintain a temperature of over 37 °C for more than 25 min. Above results indicate that the Stearic Acid@MWCNTs composite PCMs with excellent photo-thermal performance show great potential as direct solar light absorber for industrial thermal utilization, such as hot water heater, preheat burner. 3.7. Thermal property and photo-thermal performance of SA@MWCNTs composite based slurry The Stearic Acid@MWCNTs composite based phase change nanofluids is prepared by dispersing the composite into the water with tiny surfactant. The thermal conductivity of the H2O, 5 wt%, 10 wt% and 15 wt% PCMs suspension is shown in Fig. 9a. The thermal conductivity of the water increases from 0.590 to 0.668 W m−1 K−1 as the temperature range from 25 to 85 °C which is attributed to the enhancement of natural convection. When dispersing 5 wt% of the composite PCMs into the water, the thermal conductivity increases from 0.752 to 1.015 W m−1 K−1 in the same temperature range, which is 54.1% enhancement to the basefluid. The rise of the thermal conductivity as temperature increases is due to the intense natural convection of the nanofluids at high temperature. Particularly, the sharp increase between 55 °C and 75 °C is attributed to the melting of the composite PCMs. After the phase change process, the thermal conductivity at 85 °C is slightly lower than it at 75 °C but still higher than the nanofluids with equivalent mass ratio of CNTs, which is according to the Supplementary materials (S1). Similarly, the same variation trend are obtained when the composite PCMs increase to 10 wt% or 15 wt%, with the thermal conductivity enhancement of 62.0% or 72.2%. The thermal conductivity enhancement increases with the mass fraction of the composite PCMs which is attributed to the higher thermal conductivities of composite PCMs. The specific heat of the H2O, 5 wt%, 10 wt% and 15 wt% PCMs suspension is shown in Fig. 9b. The specific heat of water slightly increases from 4.205 to 4.305 J g−1 K−1 as the temperature range from 40 to 85 °C. When dispersing composite PCMs into the water, the specific heat was largely enhanced in the melting process. For instance, the 5 wt% composite PCMs based nanofluids range from 4.200 to 4.895 J g−1 K−1 in the melting process with an enhancement of 15.2%. Similarly, with the increasement of the mass fraction of the composite PCMs, the specific heat enhancement reaches up to 47.1% when the mass fraction of the composite PCMs is 15%. By contrast, the specific heat of the nanofluids with equivalent mass ratio of CNTs can be seen in the Supplementary materials (S2). The specific heat of the nanofluids without PCMs wasn't largely enhanced in the melting process. It is indicated that the heat storage capacity of the heat transfer fluid could be enhanced by adding the composite PCMs, because the composite PCMs have high energy capacity in the melting and solidifying process, as shown in Fig. 5a. Fig. 9c shows the incident photon-to-thermal conversion spectra of the composites/water based direct solar thermal collectors. During the heating process, it took 7000 s for the 5 wt% PCMs nanofluids to raise the temperature from room temperature (about 30 °C) to 75 °C. While under the same exposure time, the temperature of the 10 wt% PCMs nanofluids raise to 80 °C. That is attributed to the high mass ratio of the CNTs. Notably, the temperature of the 15 wt% PCMs nanofluids just goes up to 65 °C, which is due to reflection of the higher mass ratio of
Fig. 7. Thermal conductivity of the Stearic Acid and Stearic Acid@MWCNTs composite (a) thermal conductivity of the Stearic Acid@MWCNTs composite vis density (b).
transient plane source method, an advanced technique evolving from the hot wire method. The thermal conductivity of the Stearic Acid casting sample with the density of 847.0 kg m−3 is measured to be 0.256 W m−1 K−1 (as shown in Fig. 7a), which is very lower than the Stearic Acid@MWCNTs round blocks with the density of 900.0 kg m−3. Fig. 7b illustrates the thermal conductivity of the Stearic Acid@ MWCNTs round blocks with different packing densities. As the packing density of the blocks is increased from 900.0, 1000.0, 1100.0 and 1200.0 to 1300.0 kg m−3, their thermal conductivity accordingly increases from 7.159, 8.215, 9.134, and 10.058 to 11.154 W m−1 K−1. The relationship between the packing density (x) and the thermal conductivity (y) of the Stearic Acid@MWCNTs round blocks can be fitted into a linear equation, indicating an obvious increase in the thermal conductivity with the packing density. Note that all the Stearic Acid@MWCNTs composite PCM round blocks have much higher thermal conductivity than the Stearic Acid casting sample, owing to the integration of Stearic Acid with MWCNTs that has superior thermal conductivity. 3.6. Photo-thermal performance of SA@MWCNTs composite The photo-thermal performance of the formable Stearic Acid@ MWCNTs composite PCM containing 47 wt% of Stearic Acid with the density of 900.0 kg m−3 was conducted in the homemade equipment as shown in Fig. 1. Fig. 8 (bottom right corner) shows the photo-thermal conversion spectra of the Stearic Acid and Stearic Acid@MWCNTs composite PCM. Compared with the Stearic Acid, the temperature of the composite PCM rapidly increased upon solar irradiation. During the heating process, it took 1960 s for Stearic Acid to raise the temperature from room temperature to 75 °C but only 716 s for the Stearic Acid@ MWCNTs composite PCM. This behavior is ascribed to the function of MWCNTs as an effective photon captor and molecular heater. Simultaneously, the photo-thermal performance of the formable Stearic Acid@MWCNTs composite PCM was observed clearly through a 87
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Fig. 8. Representative thermal images about the photo-thermal performance of the Stearic Acid@MWCNTs composite.
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Fig. 9. Thermal conductivity of Stearic Acid@MWCNTs composite based nanofluids (a), Specific heat of Stearic Acid@MWCNTs composite based nanofluids (b), Photo-thermal performance of the Stearic Acid@MWCNTs composite based nanofluids (c), (d), receiver efficiency of the Stearic Acid@MWCNTs composite based nanofluids (f).
indicate that adding the encapsulated PCMs into the water can not only increase the thermal conductivity and heat capacity of the fluids, but also improve the photo-thermal performance as the heat transfer fluid in the direct solar thermal receiver. Moreover, above results show that the water based phase change slurry has the great potential to be used as low temperature heat transfer fluids in the direct absorption solar collector.
the PCMs nanofluids. Compared with the Stearic Acid@MWCNTs composite based nanofluids, without the light absorption material (MWCNTs), there is no obvious temperature rise for the basefluid. This result also proved the function of MWCNTs in this nanofluids. When dispersed the same mass fraction of CNTs into the water, the photo-thermal performance of the receiver could be almost alike (Figs. 9d, S3). This behavior is ascribed to the function of CNTs as an effective photon captor and molecular heater. To further investigate the photo-thermal performance, the receiver efficiency of above samples based direct solar thermal collectors was calculated by formula 1 according to the results of Fig. 9c. When dispersing PCMs into the fluids, their receiver efficiency could maintain in a wide temperature range, as shown in Fig. 9f. The 10 wt% PCMs based heat transfer fluid show higher receiver efficiency than the equivalent mass ratio of CNTs sample without Stearic Acid which is ascribed to the high specific heat of the PCMs based heat transfer fluid. Similarly, the receiver efficienc of 5 wt% and 15 wt% PCMs based heat transfer fluid could also maintain in a wide temperature range (Fig. 8e) and the heat transfer fluid with equivalent mass ratio of CNTs were also lower than them (Fig. S4). Particularly, the lower receiver efficienc of 15 wt% is also due to reflection of the higher mass ratio of the PCMs nanofluids. Above results
4. Conclusion A photo-driven encapsulated Stearic Acid@MWCNTs composites was successfully prepared by an simple vacuum impregnation process. In the composite, the Stearic Acid was used as the material for thermal energy storage, and MWCNTs acted as the Supporting material for improving the thermal conductivity and photo-thermal conversion performance of the composite. The melting temperature and latent heat of the composite were determined as 59.28 °C and 91.94 J g−1, respectively. The phase change characteristics of the composite are close to those of the pure Stearic Acid because there is no chemical reaction between Stearic Acid and MWCNTs in preparation of the composite. The composite with the Stearic Acid encapsulation ratio of 47% could 89
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maintain its phase transition perfectly after subjected to 200 melting–freezing cycles. Then the as-prepared Stearic Acid@MWCNTs composite was dispersed into water to form novel heat transfer fluid (with mass fraction of 5%, 10% and 15%) to be used in the direct solar thermal receiver. This new kind of fluid shows enhanced thermal conductivity (2 times), specific heat (1.5 times) and photo-thermal performance (there is no obvious temperature rise for the basefluid) as compared to the basefluids. Our work indicating that the water based phase change slurry has great potential to be used as low temperature heat transfer fluids in the solar thermal utilization systems.
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